This invention relates generally to self servowriting procedures for hard disk drives and more particularly to a non-iterative low sector rate startup procedure for writing the servo seed tracks necessary for radial propagation of the embedded servo positioning information in a data storage device.
Generally, during the hard drive manufacturing process, the hard drive is placed in a servo track writer (STW) that embeds track position information (servopattern) directly on the disk data storage surface at regular intervals in the tracks between data sectors. The hard drive servo controller is programmed into a chip located in-situ on the printed circuit board. The servo controller is updated with the actual position of the read-write head (RWH) by reading the servopattern and the desired RWH position is determined by the known storage location of the target bit. The difference between the desired and actual position is the position error signal (PES). The servo controller operates a closed servo control loop to minimize the PES by sending to a voice coil motor (VCM) the electrical current level (VCM signal value) necessary to cause an actuator arm to move the RWH into the proper radial position. The servo control loop operates in a track following mode when reading data on a track and in a track seek mode when moving to another track for more data. Operation of the servo control loop may be appreciated with reference to, for example, U.S. Pat. No. 5,381,281.
Historically, the servowriting process required special factory STWs operating under laser control in clean rooms, which quickly became a primary manufacturing bottleneck because of the capital and operating costs of STWs and because of the lengthy period (nearly an hour) devoted to the servowriting process for each hard drive. Practitioners in the hard drive manufacturing art devoted significant efforts to overcoming this servowriting bottleneck, and one of the most attractive improvement strategies is to move the servowriting process out of the clean room and into the disk drive device itself, a process denominated “self servowriting.”
Self-servowriting (SSW) is an attractive technique because it eliminates the need for costly, external positioning systems, and can be performed outside of a clean room environment. In general, this technique involves using the RWHs installed on the actuator of the disk drive in-situ to initially write the servopattern, which is thereafter used to correctly position the actuator during drive operation by a user. Disadvantageously, the open-loop in-situ servo control loop is unable to position the servopattern with the precision necessary to maintain the high track density expected in modern disk drives (100,000 tracks per inch and more).
The disk drive art is replete with proposals for overcoming this open-loop disadvantage. For example, the STW and clean room may be employed to write a few “seed” tracks to the data storage surface, which may then be later used to “self-propagate” the remainder of the servopattern under closed-loop control of the in-situ servo controller, thereby saving most of the clean-room time normally required for the servowriting process. This practice may be appreciated with reference to, for example, U.S. Pat. Nos. 5,949,603; 6,600,620; 6,631,046; and 6,977,789.
For recent disk drive track densities, the read element in the RWH on the actuator may be offset from the write element by several (5 or more) tracks. When this read-to-write element offset is large in terms of track spacing, a combination of readback amplitudes from several earlier-written servo tracks is disadvantageously needed to provide a position signal sufficiently accurate to propagate the next servo track during the self-propagation process. U.S. Pat. No. 5,757,574 proposes a basic method for overcoming this self-propagation disadvantage. Others propose writing servo bursts along a plurality of spiral paths covering the radial extent of the disk surface for later use in controlling self-propagation of a final servopattern. For example, U.S. Pat. Nos. 6,906,885; 6,943,978; 6,965,489; 6,992,852; 6,987,636; and 7,016,134 all propose adding a precise spiral servo-burst pattern to the disk surface, sometime with accompanying circular “seed tracks” for later use in self-propagating the final servopattern.
Such techniques generally require some access to the laser-controlled STW in a clean-room at some point during manufacture, which disadvantageously introduces the production bottleneck (if to a lesser degree) discussed above. Other practitioners propose techniques for correcting servopattern position errors by, for example, reading the pattern and storing measured errors in a memory on the hard drive circuit board for later use by the servo control loop. for example, reference is made to U.S. Pat. Nos. 6,937,420; and 6,061,200. Such methods are of limited efficacy at very high track densities and require additional manufacturing time and complexity.
In view of the above, the art is now replete with proposed self servowriting techniques that require no access to clean-room STWs, thereby completely eliminating that production bottleneck. Naturally, practitioners have long sought SSW techniques that can be initiated from scratch, in-situ, without a clean-room STW, but the rapid increase in track density has provided a continuing and difficult challenge for such solutions. One such proposal employs a magnetic imprinting technique to “print” a rudimentary magnetic pattern onto the disk surface during assembly for later use in self-propagating the final servopattern across the disk surface under closed-loop control of the in-situ servo controller (e.g., U.S. Pat. No. 7,099,107). Others propose various “trial and error” techniques for writing “startup” patterns under open-loop in-situ servo control conditions for use in self-propagating the final servopattern. For example, U.S. Pat. No. 5,668,679 uses an external controller to write a spiral startup pattern by controlling the in-situ electronics under open-loop conditions. The spiral pattern is read back and rewritten repeatedly until certain parameters are attained (if ever), and then the spiral pattern is used to self-propagate the final servopattern across the disk surface. This technique is complex, time-consuming and may not always ensure a useful servopattern in production conditions.
The commonly-assigned U.S. Pat. No. 6,603,627 (incorporated entirely herein by reference) describes another “trial and error” startup method for creating an initial open-loop set of concentric “seed” tracks using a compliant crashstop to control RWH movement. This method indeed avoids the use of clean room STWs, but the startup process requires a readback and rewrite of the servo seed tracks until certain parameters are obtained before self-propagating the remainder of the servopattern, so the startup process duration and final result are predictable only in the aggregate in a factory setting and may not be controllable. The commonly assigned U.S. Pat. No. 6,600,621 (incorporated entirely herein by reference) describes a method for controlling error growth during servo track self-propagation but does not consider the in situ startup problem.
Accordingly, there is still a well-known need in the art for a SSW system that eliminates trial and error from the startup procedure to provide a controlled startup process leading to a final servopattern in a predictable time. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.